The pulse wave velocity (PWV) is the speed at which a pulse travels within the arteries. It is known that PWV is a measure of arterial stiffness and also correlates with blood pressure (either Mean Arterial Pressure (MAP), or Pulse Pressure (PP)). The PWV can be calculated from the pulse transit time (PTT; the time it takes for a blood pulse to travel from the heart to a certain location), or more generally from the pulse delay (PD; the difference in PTT of two different locations on the body). PD and PTT are interchangeable for the purposes of this description, and they each may be used to provide indications in respect of PWV. The pulse delay is the difference between two Pulse Transit Times at different locations on the body originating from the same heartbeat.
Typically PWV measures are made either invasively using a catheter probe, or on the skin using tonometry. By measuring the time difference between the pulse arriving at one location on the body (such as the upper arm) and another location (such as the wrist), the velocity can be determined.
One non-invasive way to do this measurement would be to use photoplethysmograph (PPG) sensors. These measure volumetric changes of a body.
A pulse oximeter is a common example of a PPG-based sensor. The purpose of pulse oximetry is to monitor the oxygen saturation of a patient's blood. While the purpose of such a sensor is to obtain a measure of blood oxygen saturation, it also detects changes in blood volume in the skin, and thereby performs PPG sensing. By detecting changes in blood volume, a cyclic signal corresponding to the pulse is obtained. PPG sensors, such as pulse oximeters, are thus commonly used to provide a measure of the pulse rate.
A PPG sensor contains at least one LED, and one light sensor. The LED and sensor are placed such that the LED directs light into the skin of the user, which is reflected or transmitted, and detected by the sensor. The amount of reflected/transmitted light is determined by, amongst others, the perfusion of blood within the skin.
The PPG system for example includes a red LED, a near-infrared LED, and a photodetector diode. The sensor is typically configured with the LEDs and photodetector diode directly on the skin of the patient, typically on a digit (finger or toe) or earlobe.
Other places on the patient may also be suitable, including the forehead, the nose or other parts of the face, the wrist, the chest, the nasal septum, the alar wings, the ear canal, and/or the inside of the mouth, such as the cheek or the tongue.
The LEDs emit light at different wavelengths, which light is diffused through the vascular bed of the patient's skin and received by the photodetector diode. The changing absorbance at each of the wavelengths is measured, allowing the sensor to determine the absorbance due to the pulsing arterial blood alone, excluding venous blood, skin, bone, muscle, and fat for example. The resulting PPG signal may then be analyzed.
Other simpler versions of a system for obtaining PPG data may be used, including a version with a single light source of one or more wavelengths. The absorption or reflectance of the light is modulated by the pulsatile arterial blood volume and detected using a photodetector device.
In transmissive pulse oximetry, a sensor device is placed on a thin part of the patient's body. Reflectance pulse oximetry may be used as an alternative to transmissive pulse oximetry. This method does not require a thin section of the person's body and is therefore well suited to more universal application such as the feet, forehead and chest.
A basic design of a PPG sensor for example has a certain light output frequency (e.g. 128 Hz) with which the light source is pulsed. A sampling frequency of the optical sensor is higher, for example 256 Hz so that it measures during light source activation and between light source activations. This allows the system to distinguish between the emitted light from the LED and the ambient light, and thereby filter out the ambient light from the signal received during a light source pulse.
In other known proposals, PPG data can be obtained from camera images, where ambient light and/or additional light sources are used to illuminate the tissue, such as skin. PPG measurements can thus even be carried out at a distance from the tissue, where the light source and/or detector are not in contact with the tissue, such as in the case of camera-based measurements.
The PPG data may be obtained at one or more wavelengths, such as any number of wavelengths typically between 1 and 10, but more than 10 wavelengths may even be used.
Apparatus and techniques for obtaining PPG data, such as pulse oximetry data, are well known in the art and indeed many different PPG sensors are commercially available. They are for example used in devices for measuring heart rate during exercise.
This known technology could be used to measure PD. Known devices have the ability to measure and log the PPG signal for a given period of time, after which the data can be downloaded from the device, and analyzed to calculate the PD.
One limitation of these devices for the calculating PD is that it would require two units located at a distance apart on the body.
One problem that arises is the need to ensure accurate time synchronization between the two devices. Typically the internal clocks of the logging devices (i.e. the clocks used to link the output data with an actual time at which the data was recorded) are either set manually by the user, or from a computer when configuring/downloading data from the devices. The limitation of this is that if manually set the clocks are for example only accurate to the minute level, and to the second level when set via a computer. For accurate PD measurements, it is necessary to tell the time of arrival of the pulse wave to the millisecond level of accuracy. In order to attain this level of accuracy there is a need for precise synchronization between the two PPG devices.
When two PPG devices have independent clocks, they will inevitably drift relative to each other. To enable standard and independent PPG sensors to be used, the desired solution would not require electrical communication between the devices. They are for example independently started using their own on/off button. It is of course impossible in practice to manually start both devices at exactly the same time. Furthermore, it is desirable to avoid having any unnecessary processes running on the PPG devices.
While synchronization would be possible using a fully integrated system (for example based on a microprocessor clock), there is a need for a solution which enables the use of independent PPG sensors together to enable multiple synchronized measurements, for example for PD measurement.